The present invention is applicable for use in cellular radio systems employing CDMA (Code Division Multiple Access). CDMA is a multiple access method based on spread spectrum technique and it has recently been applied in cellular radio systems along with the previous FDMA and TDMA. CDMA has a plurality of advantages over prior methods, such as spectral efficiency and simplicity of frequency planning.
In the CDMA method, a narrow-band user data signal is multiplied to a relatively broad band by a spreading code having a considerably broader band than the data signal. In some known tests systems, bandwidths such as 1.25 MHz, 10 MHz and 25 MHz have been used. When multiplied, the data signal is spread to the entire band to be used. All users transmit simultaneously using the same frequency band. A separate spreading code is used on each connection between a base station and a mobile station, and the signals of different users can be distinguished from each other in the receivers on the basis of each user's spreading code.
Matched filters in receivers are synchronized with a desired signal, which they recognize by means of the spreading code. In the receiver, the data signal is restored to the original band by multiplying it again by the spreading code that was used in transmission. In an ideal case, signals multiplied by another spreading code do not correlate and are not restored to the narrow band. Accordingly, as far as the desired signal is concerned, they appear as noise. The spreading codes used in the system are preferably selected such that they are mutually orthogonal, i.e. do not correlate.
In a typical mobile phone environment, signals between a base station and a mobile station propagate along several paths between a transmitter and a receiver. Such multipath propagation is mainly caused by the signal being reflected from surrounding surfaces. Because of different propagation delays, signals that have propagated along different paths arrive at the receiver at different times. In COMA, multipath propagation may be utilized in signal reception in the same way as diversity. The receiver structure usually employed in COMA is a multibranch receiver structure, in which each branch is synchronized with a signal component that has propagated along a different path. Each branch is an individual receiver element that serves to compose and demodulate one received signal component. In a conventional CDMA receiver, the signals of the different receiver elements are combined preferably, either coherently or incoherently, whereby a high-quality signal is achieved.
COMA systems can also apply what is known as soft handover, in which case a mobile station can simultaneously communicate with several base stations by utilizing macro diversity. Consequently, during handover, mobile station connection quality remains high and the user does not detect any break in the connection.
Accordingly, interference caused by other connections to the desired connection appears in the receiver as evenly distributed noise. This is also true when a signal is examined in the angular domain according to the incoming directions of the signals detected in the receivers. Consequently, interference caused by other connections to the desired connection appears in the receiver as distributed in the angular domain, i.e. the interference is relatively evenly distributed in different incoming directions.
The capacity of the COMA, which can be measured by means of spectral efficiency, has been further improved by sectorization. In this case a cell is divided into sectors of a desired size that are serviced by directional antennas. This allows the mutual noise level caused by the mobile stations to be lowered significantly in the base station receiver. This is based on the fact that, on average, the interference is evenly distributed in the different incoming directions, the number of which can thus be reduced by means of sectorization. Sectorization can naturally be implemented in both transmission directions. The capacity gain provided by sectorization is proportional to the number of sectors.
A sectorized cell may also utilize a special form of soft handover, i.e. softer handover, wherein a mobile station performs handover from one sector to another by communicating simultaneously with both sectors. Even though soft handover improves connection quality, and sectorization increases system capacity, the movement of the mobile stations naturally causes them to perform several handovers from one sector to another. This loads the processing capacity of the base station controller. Several soft handovers also bring about a situation where several mobile stations communicate simultaneously with more than one sector (usually two), whereby the capacity gain provided by sectorization is lost, since a mobile station signal is audible within a wide sector.
The multiple access interference of the CDMA systems has also been reduced by means of different known multiple access interference cancellation (IC) methods and multi-user detection (MUD). These methods are best suited for reducing the interference originating from the user's own cell, and system capacity can thus be doubly increased compared with a system implemented without interference cancellation. However, these methods do not significantly improve the size of the coverage area of the base station as compared with the prior art. IC/MUD techniques are also complicated to implement, and have therefore mainly been designed for the uplink direction.
Furthermore, a method that is known as SDMA (Space Division Multiple Access) has been designed wherein users are distinguished from each other on the basis of location. This is performed by adjusting the beams of the receiver antennas at the base station in the desired directions according to the locations of the mobile stations. For this purpose, adaptive antenna arrays, i.e. phased antennas, and processing of a received signal are used, by means of which the mobile stations are tracked.
The utilization of SDMA with CDMA provides several advantages over prior methods, e.g. sectorization. If the sector beams are narrowed in sectorization in order to increase spectral efficiency, the number of handovers to be performed from one sector to another also increases. This in turn excessively increases the calculation capacity required in the base station controller.
For the application of SDMA, the background art is illustrated in A. F. Naguib, A. Paulraj: Performance of CDMA Cellular Networks With Base-Station Antenna Arrays (Proc. International Zürich Seminar on Digital Communications, pp. 87 to 100, Zürich, Switzerland, March 1994), which is incorporated herein by reference. In SDMA, a signal is received by means of an antenna array, and the received signal is shaped by means of digital signal processing in such a way that the directivity patterns of the antennas are suitable for the stages following the shaping in the receiver. In prior art solutions, the received signal is shaped in order to maximize the signal-to-interference ratio of the desired signal. In other words, the received signal is shaped in such a way that the directivity pattern of the antenna array minimizes the interference caused by the other connections in the desired signal. In the solution according to the aforementioned reference, each detected signal component is subjected to individual beam shaping. i.e. the impulse response must be known before the shaping.